In the past, solar generators aimed at exploiting the high efficiency of multi-junction solar cells to generate electricity typically employed many small solar focusing optical systems for each individual photovoltaic cell. Such generators were deficient in that the packaged assemblies of numerous optical systems and cells were both large and complex, and consequently suffered from a relatively high cost that made such solar generators uncompetitive with alternative methods of generating electricity. Such generators also required large, unique facilities for their manufacture, and were expensive to transport from the factory to an installation site.
Some previous designs of solar generators have been disclosed that use single large reflectors to power arrays of multi-junction photovoltaic cells. U.S. Patent Application Publication No. 2011/0168234, by John Lasich, titled “Photovoltaic Device for a Closely Packed Array,” describes a solar generator with a densely-packed array of solar cells near the focus of a large paraboloidal reflector dish. Planar mirrors are arranged around the perimeter of a densely packed array. One drawback of the proposed configuration by Lasich is that no provision is made to direct light away from the light-insensitive electrical connections on the front surface of the arrayed cells, causing losses and reduced efficiency. Another drawback is that the illumination is not uniformly distributed across the array, causing loss of power when individual cells are connected in series. Yet another drawback is that small mispointing of the optical axis away from the sun would cause the illumination to become more uneven, further reducing power output. Lasich proposes the use of stiff, heavy trackers to mitigate this problem by maintaining accurate pointing, but such trackers drive up cost.
U.S. Pat. No. 8,350,145, by Roger P. Angel, titled “Photovoltaic Generator with a Spherical Imaging Lens for Use with a Paraboloidal Solar Reflector,” uses a spherical ball lens at the focus of a paraboloidal dish reflector. The lens stabilizes the light against mispointing at the image of the dish reflector, formed on a concave surface, and tiled with tapered optical funnels. At each funnel output, the light is distributed into discrete square regions, with a photovoltaic cell located at each region.
However, because the apparatus disclosed in U.S. Pat. No. 8,350,145 relies on spherical symmetry to realize equal apportionment of sunlight to a plurality of photovoltaic cells arranged in a concave array, the cells and optical funnels are configured in a concave array. In practice, the manufacturing costs involved in making curved reflecting surfaces and supporting structures for the concave array of photovoltaic cells have been relatively high. In addition, the lens itself is preferably made as a full sphere (ball lens), and both the optical funnels and the photovoltaic cells are deployed on concentric concave spherical surfaces. Some embodiments use photovoltaic cells of many different shapes and sizes to tile the spherical surface, and in practice, this added complexity has increased costs. Some embodiments use identical square cells, but complex funnel shapes are configured to fit together seamlessly to tile a spherical surface at their input, and to match the square cell dimension at their output. In practice, such embodiments have been relatively expensive to manufacture, because the funnels are manufactured with many different odd shapes to fit together, and the individual reflective surfaces of a funnel, instead of being flat, are twisted to bring the light from an odd entrance shape to a square output to match the square photovoltaic cell. In addition, providing the funnel surfaces with high specular reflectance, and subsequent coating for very high reflectivity, tend to be more expensive to manufacture.
Mounting photovoltaic cells to conform to a spherical surface may be problematic. If individual flat photovoltaic cells are to be mounted individually on electrically insulating but thermally conductive substrates, and such substrates to be attached to a concave, faceted surface, with the facets tangent to a sphere, the mounting process is further complicated by the additional requirement for transfer of high flows of both heat and electricity from the substrates.
In some prior designs, compensation for shadowing of a primary mirror by a central assembly of secondary optics and any supporting structure is achievable only by eliminating partly blocked cells from a series-connected chain. This may waste light, and consequently lead to reduced efficiency and power output.
It follows that many prior designs have suffered from relatively high manufacturing costs. In addition, some prior designs may have inevitable light blockages that break the continuous sunlight beam from a primary reflector, and as a result, may cause current imbalances and reduced power output. There is therefore room for improvement.